Copolymers are of significant interest for a diverse range of applications, as they can impart the material with properties of both individual polymers as well as new properties of the combinations. Thus far a significant body of research has been carried out on linear diblock and triblock copolymers, and behaviour of these materials is relatively well understood. While comb-like or graft copolymer architectures have provided access to new materials, in general, there are relatively few examples involving these more complicated polymer architectures and their properties such as on surfaces, films, and their assemblies in aqueous solution are less well understood. However, they present interesting properties including the ability to finely tune their architectures by adjusting the grafting densities and relative chain lengths.
In recent years, much research has emerged to suggest that polyisobutylene (PIB)-based materials are highly promising for a number of biomedical applications (Puskas et al., Biomacromolecules 2004, 5, 1141-1154 and J. Polym. Sci. Part A: Polym. Chem. 2004, 42, 3091-3109). For example, PIB-polystyrene (PS) triblock copolymers are currently being used as a drug eluting coating on TAXUS® vascular stents (Pinchuk et al., Biomaterials 2008, 29, 448-460.). Copolymers of PIB with hydrophilic polymers such as poly(N,N-dimethylacrylamide) or poly(ethylene oxide) (PEO) have been used to form membranes that can encapsulate cells while allowing the exchange of oxygen, nutrients, and secreted proteins such as insulin across the membrane (Isayeva et al., Biomaterials 2003, 24, 3483-3491). However, optimization of the polymer chemistry and properties is still critical for many applications. For example, when PIB-PS was explored as a potential implant material in the urinary tract, significant attachment of uropathegenic species such as E. coli 67 was observed, indicating that the surface properties of the polymer were not ideal for this application (Cadieux et al., Colloids Surf, B 2003, 28, 95-105).
The incorporation of PEO into PIB-based materials is of particular interest as it is known to confer resistance of the surface to proteins, which is a significant asset for biomedical devices and implants that often otherwise undergo rapid biofouling (Cadieux et al., Colloids Surf, B 2003, 28, 95-105; Harris, M. J., Poly(ethylene glycol) Chemistry: Biotechnical and Biomedical Applications. Plenum Press: New York, 1992; Andrade et al., Hydrophilic Polymers. In Glass, J. E., Ed. American Chemical Society: Washington D.C., 1996; Vol. 248, pp 51-59; Leckband et al., J. Biomater. Sci. Polym. Ed. 1999, 10, 1125-1147; Hoffman, A. S. J. Biomater. Sci. Polym. Ed. 1999, 10, 1011-1014; and Krishnan et al., J. Mater. Chem. 2008, 18, 3405-3413).
The grafting of PEO onto PIB is also of significant interest due to enhanced mechanical properties, increased wettability, microphase separation, and emulsifying properties observed in these polymers.
PIB-PEO linear block copolymers have previously been reported, but their syntheses are not straightforward as they typically involve a living cationic polymerization to form an end-functionalized PIB block, followed by the coupling of PEO to the terminus using this functionality (Kennedy, J. P.; Ivan, B., Designed Polymers by Carbocationic Macromolecular Engineering Theory and Practice. Hanser: New-York, 1992; and Kaszas et al., J. Macromol. Sci., Chem. 1989, A26, 1099-1114). For example, Gao and Kops coupled phenol-terminated PIB with tosylated PEO (Gao, B.; Kops, J. Polym. Bull. 1995, 34, 279-286), Roony reacted the same end-functionalized PIB with PEO by isocyanate chemistry (Rooney, J. M. J. Polym. Sci. Part A: Polym. Chem. 1981, 19, 2119-2122), and Kurian et al. used a coupling between silane functionalized PIB and allyl functionalized PEO (J. Polym. Sci. Part A: Polym. Chem. 2000, 38, 3200-3209). However, each of these examples involved some degree of side reactions and/or low yields.
The grafting of PEO onto butyl rubber, a copolymer of isobutylene and small percentages of isoprene, has also been explored, but has been limited by challenges associated with solubility, low reactivity, and purification. Kohjiya and coworkers have prepared butyl rubber-PEO graft copolymers by the reaction of chlorinated and brominated butyl rubber with the potassium salt of PEO monomethyl ether (m-PEO), where the reactions were performed from 80° C. to 110° C. (J. Polym. Sci. Part. A Polym. Chem. 1993, 31, 2437-2444). Whitney, Parent and coworkers have explored the grafting of PEO onto bromobutyl using both the potassium salt of m-PEO as well as a carboxylate derivative, wherein limitations imposed by the molecular weight of the PEO were mentioned and the purified copolymers contained substantial amounts of conjugate diene (dehydrobromination side reaction limited reaction yield) because the reaction was performed at 115° C. with several equivalents of KOH (Eur. Polym. J. 2007, 43, 4619-4627). An additional example by Parent and coworkers involved the use of an acid terminated PEO of MW 700 g/mol, wherein reactions were performed at 90° C. Overall, the above reaction conditions are relatively harsh (extensive heating/strongly basic conditions) and there are significant degrees of side reactions limiting the reaction yields and resulting PEO content. Although recent catalytic approaches appear to be more promising, incomplete couplings as well as side reactions such as eliminations to the conjugated diene were typically observed (McLean et al., Ind. Eng. Chem. Res. 2009, 48, 10759-10764; and Parent et al., Eur. Polym. J. 2010, 46, 702-708).